A study of aero engine fan flutter at high rotational speeds using holographic interferometry

Aero-elastic instability is often a constraint on the design of modern high by-pass rat.io aero engines. Unstalled supersonic flutter is an instability which can be encountered in shrouded fans, in which mechanical vibrations give rise to unsteady aerodynamic forces which couple further energy into the mechanical vibration. This phenomenon is particularly sensitive to the deflection shape of the mechanical vibration, A detailed measurement of the vibrational deflection shape of a test fan undergoing supersonic unstalled flutter was sought by the author. This measurement was required in order to assess the current theoretical understanding and modelling of unstalled fan flutter, The suitability of alternative techniques for this measurement was assessed, Pulsed holographic interferometry was considered optimum for this study because of its full field capability, large range of sensitivity, high spatial resolution and good accuracy. A double pulsed holographic system, employing a rnirror~Abbe image rotator, was built specifically for this study, The mirror-Abbe unit was employed to rotate the illuminating beam and derotate the light returned from the rotating fan. This therefore maintained correlation between the two resultant holographic images. The holographic system was used to obtain good quality interferograms of the 0.86m diameter test fan when rotating at speeds just under 10 000rpm and undergoing unstalled flutter. The resultant interferograms were analysed to give the flutter deflection shape of the fan. The study of the fan in flutter was complemented by measurement of the test fan's vibrational characteristics under non-rotating conditions. The resultant experimental data were in agreement with the current theoretical understanding of supersonic unstalled fan flutter. Many of the assumptions employed in flutter prediction by calculation of unsteady work were experimentally verified, The deflection shapes of the test fan under non-rotating and flutter conditions were compared with those predicted by a finite element model of the structure and reasonably good agreement was obtained.